During the past year we have been recording intracellularly in the cat's leteral geniculate nucleus (LGN) with micropipette electrodes filled with the enzyme horseradish peroxide (HRP). With this technique one can characterize a cell in terms of its physiological properties and its receptive-field organization. Then it can be labelled with the intracellular marker for future histological examination. During the coming year we shall utilize this approach in the study of cells in the monkey's LGN. We shall first characterize the different cell types and then compare this information with their morphology and location in the geniculate. There are preliminary indications that it will be possible to follow an HRP-filled geniculate axon along its entire length to its terminations in the monkey's visual cortex. This technique will allow us to determine the destinations of the axons from the various classes of geniculate cells.